The present invention relates to electronics and, more particularly, to phase-control systems for controlling the power delivered to a load. A major objective of the present invention is to provide for economical protection of a forward-phase-control dimmer or similar AC power controller from damage due to an overcurrent condition.
As the world's resources are being depleted, energy conservation increases in importance. One approach to conserving energy is to supply less power to a device than it is rated for when less than full power operation is required. For example, dimmers can be used to operate lights to provide only the light level desired at a given time. In contrast, on/off switches require that a light be operated only at its full rated power. Since dimmers not only save energy but also provide more control over the light level, they are generally more desirable than on/off switches. However, economic considerations at installation favor on/off switches.
The power from an AC power source delivered to a load can be controlled by selecting the portion (conduction angle) of an AC voltage half cycle that the AC power is coupled to the load. This power control technique is known as "phase control"--since power is coupled at one voltage phase angle and decoupled at another voltage phase angle. (See Phase Control Using Thyristors, Teccor Electronics, Inc., undated publication.)
"Forward-phase control" (FPC) involves coupling within a voltage half cycle and decoupling at the next zero crossing. The timing for the switching can be provided by a microcontroller or other timing device along with phase-tracking circuitry. The switching can be done using thyristors such as silicon-controlled rectifiers (SCRs) or triacs. A thyristor is activated by a trigger signal, and deactivated automatically at the next voltage zero crossing.
"Reverse-phase control" (RPC) involves coupling at a zero crossing and decoupling within the ensuing half voltage cycle. As with FPC, the timing for the phase control can be provided by a microcontroller. However, RPC is not implemented with thyristors since they cannot be easily or economically switched off by a signal at a control input. However, transistors, e.g., bipolar transistors, can be used as RPC current switches.
One advantage of RPC is the effectiveness with which overload protection can be implemented. The transistor is turned on when the voltage, and thus the current, is zero. The magnitude of the current can be monitored as it increases. If it approaches an overload threshold, the switch can be turned off before it is destroyed.
This monitoring approach is less applicable to thyristor-based FPC systems for two reasons. In the first place, the peak current is often (especially when the triggering is at or past a voltage peak) reached almost instantaneously, so there is little time to react before damage is done to the thyristor. In the second place, even if the damage can be anticipated, it is not in general possible to shut down a thyristor immediately; in general, a thyristor cannot be shut down until the next zero crossing. Thus, in a thyristor-based dimmer, overload protection typically does not begin until the next half-cycle. In many cases, the damage is already done.
Despite the problems with overload protection, FPC dimmers dominate the low-end of the dimmer market because they work with a wider variety of loads and because they cost less. FPC "dimmers" are better suited than are RPC dimmers for controlling inductive loads, such as fans and other motor-driven appliances. (Some systems test for inductive loads and select between FPC and RPC accordingly; see Steven B. Carlson, "Testing IPS Dimmer Performance", ROSCO/Entertainment Technology, 1996, published on the World-Wide Web.) Since the function performed by a single relatively inexpensive triac in an FPC system requires a pair of relatively expensive bipolar transistors in an RPC system, component costs favor FPC systems.
Because of the relatively low cost of FPC dimmers, replacement of wall switches with FPC dimmers is a popular "home-improvement" project. However, a significant percentage of non-professional installations establish an inadvertent short circuit between the load connection and AC neutral or protective earth ground. In other cases, an overload can be created when a device is added in parallel to a load already being driven using the dimmer. When a triac is activated in such cases, an excessive current can be generated that leaves the triac in a permanently conductive state. Often, the damaged dimmer is returned from the place of purchase. Such returns result in a cost that must be reflected in the original purchase price.
What is needed is a thyristor-based FPC system with more effective overload protection. In particular, it is desirable that an FPC system not be destroyed upon activation after an installation that establishes a short circuit. This more effective overload protection would reduce returns and thus the cost of dimmers. This would increase consumer satisfaction with such devices, and increase their prevalence. An increased prevalence of such dimmers would reduce energy consumption--benefiting both the user with reduced costs and society with the concomitant energy conservation.